Yoshiki Uemura

The high-molecular-weight serine proteinase inhibitors (serpins) are restricted, generally, to inhibiting proteinases of the serine mechanistic class. However, the viral serpin, cytokine response modifier A, and the human serpins, antichymotrypsin and squamous cell carcinoma antigen 1 (SCCA1), inhibit different members of the cysteine proteinase class. Although serpins employ a mobile reactive site loop (RSL) to bait and trap their target serine proteinases, the mechanism by which they inactivate cysteine proteinases is unknown. Our previous studies suggest that SCCA1 inhibits papain-like cysteine proteinases in a manner similar to that observed for serpin-serine proteinase interactions. However, we could not preclude the possibility of an inhibitory mechanism that did not require the serpin RSL. To test this possibility, we employed site-directed mutagenesis to alter the different residues within the RSL. Mutations to either the hinge or the variable region of the RSL abolished inhibitory activity. Moreover, RSL swaps between SCCA1 and the nearly identical serpin, SCCA2 (an inhibitor of chymotrypsin-like serine proteinases), reversed their target specificities. Thus, there were no unique motifs within the framework of SCCA1 that independently accounted for cysteine proteinase inhibitory activity. Collectively, these data suggested that the sequence and mobility of the RSL of SCCA1 are essential for cysteine proteinase inhibition and that serpins are likely to utilize a common RSL-dependent mechanism to inhibit both serine and cysteine proteinases.

An elevation in the circulating level of the squamous-cell carcinoma antigen (SCCA) can be a poor prognostic indicator in certain types of squamous-cell cancers. Total SCCA in the circulation comprises 2 nearly identical, approximately 45 kDa proteins, SCCA1 and SCCA2. Both proteins are members of the high-molecular weight serine proteinase inhibitor (serpin) family with SCCA1 paradoxically inhibiting lysosomal cysteine proteinases and SCCA2 inhibiting chymotrypsin-like serine proteinases. Although SCCA1 and SCCA2 are detected in the cytoplasm of normal squamous epithelial cells, neither serpin is detected normally in the serum. Thus, their presence in the circulation at relatively high concentrations suggests that malignant epithelial cells are re-directing serpin activity to the fluid phase via an active secretory process. Because serpins typically inhibit their targets by binding at 1:1 stoichiometry, a change in the distribution pattern of SCCA1 and SCCA2 (i.e., intracellular to extracellular) could indicate the need of tumor cells to neutralize harmful extracellular proteinases. The purpose of our study was to determine experimentally the fate of SCCA1 and SCCA2 in squamous carcinoma cells. Using subcellular fractionation, SCCA-green fluorescent fusion protein expression and confocal microscopy, SCCA1 and SCCA2 were found exclusively in the cytosol and were not associated with nuclei, mitochondria, lysosomes, microtubules, actin or the Golgi. In contrast to previous reports, metabolic labeling and pulse-chase experiments showed that neither non-stimulated nor TNFalpha/PMA-stimulated squamous carcinoma cells appreciably secreted these ov-serpins into the medium. Collectively, these data suggest that the major site of SCCA1 and SCCA2 inhibitory activity remains within the cytosol and that their presence in the sera of patients with advanced squamous-cell carcinomas may be due to their passive release into the circulation.

Resveratrol (3,4',5-trihydroxystilbene) is a phytoalexin found in grapes and other food products that can prevent cancer. We studied the in vitro biological activity of this compound by examining its effect on proliferation and inducing apoptosis in three lung cancer cell lines (A549, EBC-1, Lu65). Resveratrol inhibited the growth of A549, EBC-1 and Lu65 lung cancer cells by 50% (ED50) at concentrations between 5-10 microM. We also examined the combined effects in these cells of resveratrol and paclitaxel, an essential chemotherapeutic agent against lung cancer. Although simultaneous exposure to resveratrol plus paclitaxel did not result in significant synergy, resveratrol (10 microM, 3 days) significantly enhanced the subsequent antiproliferative effect of paclitaxel. In addition, resveratrol as well as paclitaxel induced apoptosis in EBC-1 and Lu65 cells, as measured by TUNEL and caspase assays, as well as flow cytometry. Resveratrol (10 microM, 3 days) similarly enhanced the subsequent apoptotic effects of paclitaxel. We examined the effects of resveratrol and paclitaxel on levels of p21waf1, p27kip1, E-cadherin, EGFR and Bcl-2 in EBC-1 cells. Resveratrol (10 microM, 3 days) prior to paclitaxel induced p21waf1 expression approximately 4-fold. These results suggest that resveratrol may be a promising alternative therapy for lung cancer and that lung cancer cells exposed to resveratrol have a lowered threshold for killing by paclitaxel.